Use Of Ionization Chambers Research Articles

Characterization of a 3D prototyping semi-automated filter holder system for RQR radiation qualities

Characterizing radiation measurement systems, such as diagnostic radiology systems for RQR and RQA, is essential for ensuring reliable results in standardized instrument calibration laboratories. Quality control of dosimetry devices, such as patient dose calibrators (PDCs), a reference meter, is crucial for ensuring accurate measurements. This study demonstrates the use of spectrometry and ionization chambers to characterize a set of semi-automated 3D prototyping filter holder systems, developed in a previously project, designed for applications such as Kerma-Area Product (KAP) meter calibration. The set of 3D prototypes were adjusted to a filter wheel and characterized using an X-ray spectrometer, followed by an ionization chamber, according to international standards. The characterization of radiation qualities from 50 to 150 kV in diagnostic radiology was performed using the substitution method: first with an ionization chamber, then with a PDC. The PDC was calibrated for KAP and Air Kerma quantities, considering the conditions of the X-ray system and factors such as temperature, humidity, and pressure. Spectrometric analysis and the use of an ionization chamber were effective in determining these parameters. Spectral characterization allowed for accurate measurements to establish proper quality control, then it is possible to use the PDC calibrated for calibration procedures for KAP meters. Future studies intend to compare these results with computational simulations using the TOPAS Monte Carlo code to validate the calibration method.

VHEE beam dosimetry at CERN Linear Electron Accelerator for Research under ultra-high dose rate conditions

The aim of this work is the dosimetric characterization of a plane parallel ionization chamber under defined beam setups at the CERN Linear Electron Accelerator for Research (CLEAR). A laser driven electron beam with energy of 200 MeV at two different field sizes of approximately 3.5 mm FWHM and approximately 7 mm FWHM were used at different pulse structures. Thereby the dose-per-pulse range varied between approximately 0.2 and 12 Gy per pulse. This range represents approximately conventional dose rate range beam conditions up to ultra-high dose rate (UHDR) beam conditions. The experiment was based on a water phantom which was integrated into the horizontal beamline and radiochromic films and an Advanced Markus ionization chamber was positioned in the water phantom. In addition, the experimental setup were modelled in the Monte Carlo simulation environment FLUKA. In a first step the radiochromic film measurements were used to verify the beamline setup. Depth dose distributions and dose profiles measured by radiochromic film were compared with Monte Carlo simulations to verify the experimental conditions. Second, the radiochromic films were used for reference dosimetry to characterize the ionization chamber. In particular, polarity effects and the ion collection efficiency of the ionization chamber were investigated for both field sizes and the complete dose rate range. As a result of the study, significant polarity effects and recombination loss of the ionization chamber were shown and characterized. However, the work shows that the behavior of the ionization chamber at the laser driven beam line at the CLEAR facility is comparable to classical high dose-per-pulse electron beams. This allows the use of ionization chambers on the CLEAR system and thus enables active dose measurement during the experiment. Compared to passive dose measurement with film, this is an important step forward in the experimental equipment of the facility.

The Practice of Additive Manufacturing for Estimation of Average Absorbed Dose in Clinical Proton Beams

Abstract: There is a necessary requirement to provide precise dosimetry measurements for radiotherapy and radiobiological studies in the proton beams. The most common practice nowadays to obtain the dose distribution is the use of ionization chambers. However, for many needs, it is also required to estimate an average absorbed dose in the target, while the targets themselves might have complex geometries and large volumes. One of the recent successful method for such measurement is the chemical dosimetry using FBX solutions coupled with the additive manufacturing, which can ensure the accurate representation of complex target geometries. In this study, we chose an optimal chemically neutral material for 3D printing that is not reacting with any of FBX compounds, manufactured the sealed waterproof target models with complex geometry and performed preliminary measurements of the average absorbed dose in a number of volumes representing the different shapes of the targets. The obtained results strongly confirm the possibility of the use of the presented technology for practical dosimetry of proton beams.

SU‐F‐T‐293: Experimental Comparisons of Ionization Chambers with Different Volumes for CyberKnife Delivery Quality Assurance

Purpose:To evaluate the practicality use of ionization chambers with different volumes for delivery quality assurance of CyberKnife plans,Methods:Dosimetric measurements with a spherical solid water phantom and three ionization chambers with volumes of 0.13, 0.04, and 0.01 cm3 (IBA CC13, CC04, and CC01, respectively) were performed for various CyberKnife clinical treatment plans including both isocentric and nonisocentric delivery. For each chamber, the ion recombination correction factors <I>Ks</I> were calculated using the Jaffe plot method and twovoltage method at a 10‐cm depth for a 60‐mm collimator field in a water phantom. The polarity correction factors <I>Kpol</I> were determined for 5–60‐mm collimator fields in same experimental setup. The measured doses were compared to the doses for the detectors calculated using a treatment planning system.Results:The differences in the <I>Ks</I> between the Jaffe plot method and two‐voltage method were −0.12, −0.02, and 0.89% for CC13, CC04, and CC01, respectively. The changes in <I>Kpol</I> for the different field sizes were 0.2, 0.3, and 0.8% for CC13, CC04, and CC01, respectively. The measured doses for CC04 and CC01 were within 3% of the calculated doses for the clinical treatment plans with isocentric delivery with collimator fields greater than 12.5 mm. Those for CC13 had differences of over 3% for the plans with isocentric delivery with collimator fields less than 15 mm. The differences for the isocentric plans were similar to those for the single beam plans. The measured doses for each chamber were within 3% of the calculated doses for the non‐isocentric plans except for that with a PTV volume less than 1.0 cm3.Conclusion:Although there are some limitations, the ionization chamber with a smaller volume is a better detector for verification of the CyberKnife plans owing to the high spatial resolution.

Investigations of the applicability of a new accountancy tool in a closed tritium loop

A commonly used activity monitoring method for tritium accountancy and process monitoring in tritium technology is ionization counting. Despite the wide use of ionization chambers (IC), they have several drawbacks like a strong gas species and pressure dependency. Furthermore, if compact systems are needed, there is also the necessity for process gas pressures >10kPa. To encounter these drawbacks, the TRitium Activity Chamber Experiment (TRACE) has been developed at the Tritium Laboratory Karlsruhe (TLK) as a compact tritium monitor based on the beta induced X-ray spectrometry (BIXS) principle.TRACE can be used as an accountancy tool in tritium-processing facilities like the KArlsruhe TRItium Neutrino (KATRIN) experiment. In contrast to ICs TRACE shows a linear response to pressure changes up to approx. 1kPa. The results of performed flow-through measurements confirm that TRACE is a complement for ICs in the low-pressure regime. Furthermore the gas species dependency of TRACE is investigated both with tritium measurements and with Monte Carlo simulations.

Partial kilovoltage cone beam computed tomography, complete kilovoltage cone beam computed tomography, and electronic portal images for breast radiation therapy: A dose-comparison study

PurposeThe purpose of this study was to compare absorbed dose with the treated breast and organs at risks (OARs) with weekly image guidance using electronic portal imaging (EPI), complete kilovoltage cone beam computed tomography (kV CBCT), and partial kV CBCT. Methods and materialsUsing a thorax female phantom, we determined absorbed doses to treated and contralateral breast, ipsilateral and contralateral lung, heart, and skin for tangential EPI, complete kV CBCT, and partial kV CBCT. Doses were measured by use of ionization chambers and compared with treatment planning system calculations. With simulation of breast tangential irradiation to a standard dose of 50 Gy in 25 fractions, dose to each organ was measured for each image guidance technique. ResultsUse of weekly EPI was associated with a significantly increased dose to the treated breast compared with weekly complete and partial kV CBCT (4.44 ± 0.04 vs 1.00 ± 0.07 vs 0.576 ± 0.003 cGy, respectively). Dose to the contralateral breast, ipsilateral and contralateral lung, heart, and contralateral skin was lower with EPI than with either complete or partial kV CBCT (0.042 ± 0.004 vs 0.36 ± 0.01 vs 0.23 ± 0.01 cGy, 0.06 ± 0.04 vs 0.42 ± 0.02 vs 0.31 ± 0.01 cGy, 0.004 ± 0.002 vs 0.29 ± 0.01 vs 0.22 ± 0.01 cGy, 0.03 ± 0.08 vs 0.36 ± 0.02 vs 0.25 ± 0.01 cGy, and 0.20 ± 0.02 vs 0.80 ± 0.06 vs 0.40 ± 0.03 cGy, respectively). Compared with complete CBCT, the use of partial CBCT allowed dose reductions of 42%, 37%, 27%, 24%, and 28% to the ipsilateral breast, contralateral breast, ipsilateral lung, contralateral lung, and heart, respectively. Additional dose from weekly CBCT was significantly lower than treatment-related scatter dose for all OARs. ConclusionsUse of CBCT was associated with decreased dose to ipsilateral breast and increased dose to all OARs compared with EPI. Significant dose reduction can be achieved with the use of partial CBCT, while generally maintaining image quality.

A new beam loss detector for low-energy proton and heavy-ion accelerators

The Facility for Rare Isotope Beams (FRIB) to be constructed at Michigan State University shall deliver a continuous, 400kW heavy ion beam to the isotope production target. This beam is capable of inflicting serious damage on accelerator components, e.g. superconducting RF accelerating cavities. A Beam Loss Monitoring (BLM) System is essential for detecting beam loss with sufficient sensitivity and promptness to inform the machine protection system (MPS) and operations personnel of impending dangerous losses. Radiation transport simulations reveal shortcomings in the use of ionization chambers for the detection of beam losses in low-energy, heavy-ion accelerators. Radiation cross-talk effects due to the folded geometry of the FRIB LINAC pose further complications to locating specific points of beam loss. We propose a newly developed device, named the Loss Monitor Ring (LMR11Loss Monitor Ring.), to be implemented upstream of each FRIB cryomodule, as part of the direct loss monitoring system to fulfill the needs of machine protection.

The clinical impact of detector choice for beam scanning.

Recently, the developers of Eclipse have recommended the use of ionization chambers for all profile scanning, including for the modeling of VMAT and stereotactic applications. The purpose of this study is to show the clinical impact caused by the choice of detector with respect to its ability to accurately measure dose in the penumbra and tail regions of a scanned profile. Using scan data acquired with several detectors, including an IBA CC13, a PTW 60012, and a Sun Nuclear EDGE Detector, three complete beam models are created, one for each respective detector. Next, using each beam model, dose volumes are retrospectively recalculated from actual anonymous patient plans. These plans include three full‐arc VMAT prostate plans, three left chest wall plans delivered using irregular compensators, two half‐arc VMAT lung plans, three MLC‐collimated static‐field pairs, and two SBRT liver plans. Finally, plans are reweighted to deliver the same number of monitor units, and mean dose‐to‐target volumes and organs at risk are calculated and compared. Penumbra width did not play a role. Dose in the tail region of the profile made the largest difference. By overresponding in the tail region of the profile, the 60012 diode detector scan data affected the beam model in such a way that target doses were reduced by as much as 0.4% (in comparison to CC13 and EDGE data). This overresponse also resulted in an overestimation of dose to peripheral critical structure, whose dose consisted mainly of scatter. This study shows that, for modeling the 6 MV beam of Acuros XB in Eclipse Version 11, the choice to use a CC13 scanning ion chamber or an EDGE Detector was an unimportant choice, providing nearly identical models in the treatment planning system.PACS number: 87.55.kh

A mammographic phantom to measure mean glandular dose by thermoluminescent dosimetry

We have designed a phantom to evaluate mean glandular dose (MGD) as part of the regulatory dosimetry control for mammographic equipment. The phantom is constituted by TLD-100 thermoluminescent dosemeters (TLDs) inserted within semicircular plates of acrylic. Different groups of TLDs are used to determine entrance surface air kerma and half-value layer (HVL). Calibration of both tasks has been performed using a Senographe 2000D system and an ionization chamber. The phantom has been tested in five clinical systems. The HVL and MGD obtained by this method agree, on average, within 3%, with those from standard procedures based on the use of ionization chambers. The phantom MGD measurements have a combined uncertainty better than 10% (k = 1).

The determination of beam quality correction factors: Monte Carlo simulations and measurements

Modern dosimetry protocols are based on the use of ionization chambers provided with a calibration factor in terms of absorbed dose to water. The basic formula to determine the absorbed dose at a user's beam contains the well-known beam quality correction factor that is required whenever the quality of radiation used at calibration differs from that of the user's radiation. The dosimetry protocols describe the whole ionization chamber calibration procedure and include tabulated beam quality correction factors which refer to 60Co gamma radiation used as calibration quality. They have been calculated for a series of ionization chambers and radiation qualities based on formulae, which are also described in the protocols. In the case of high-energy photon beams, the relative standard uncertainty of the beam quality correction factor is estimated to amount to 1%. In the present work, two alternative methods to determine beam quality correction factors are prescribed—Monte Carlo simulation using the EGSnrc system and an experimental method based on a comparison with a reference chamber. Both Monte Carlo calculations and ratio measurements were carried out for nine chambers at several radiation beams. Four chamber types are not included in the current dosimetry protocols. Beam quality corrections for the reference chamber at two beam qualities were also measured using a calorimeter at a PTB Primary Standards Dosimetry Laboratory. Good agreement between the Monte Carlo calculated (1% uncertainty) and measured (0.5% uncertainty) beam quality correction factors was obtained. Based on these results we propose that beam quality correction factors can be generated both by measurements and by the Monte Carlo simulations with an uncertainty at least comparable to that given in current dosimetry protocols.

WE‐E‐213A‐04: Practical Aspects of Electron Beam Dosimetry

Electron beam dosimetry can be a deceptively simple endeavor. On one hand, the process of measuring and using unrestricted electron cone factors is remarkably straight forward. However, making measurements in restricted fields especially at extended treatment distances can prove to be quite a challenge, especially if the field is extremely narrow. Ionization chamber correction coefficients for cylindrical and plane‐parallel chambers can depend on many factors and ensuring that the correct factors are applied for measurements at depth in water or solid phantoms can be confusing.This talk will cover proper measurement set up and technique, problems and pitfalls associated with detector selection for various electron measurement situations, chamber response characteristics that should be known and determined before use for electron dosimetry, and efficient techniques for performing routine commissioning measurements and special dosimetry for electron beams. The influence of TG‐51 calibration protocol on routine clinical measurements will also be covered along with some clinical techniques where electron beam measurements are particularly challenging and interesting.Learning Objectives:1. Ionization chamber response characteristics in electron beams2. Selection of detectors for measurements in electron beams3. Use of ionization chambers and other detectors in electron beams measurements4. Special measurement situations: restricted field measurements, measurements at extended distances, measurements in non‐water phantoms5. Clinical situations involving special electron beam measurements

Modelling ionization chamber response to nonstandard beam configurations

Novel technologies aiming at improving target dose coverage while minimising dose to organs at risk use delivery of radiation fields that significantly deviate from reference conditions defined in protocols such as TG-51 and TRS-398. The use of ionization chambers for patient-specific quality assurance of these new delivery procedures calibrated in reference conditions increases the uncertainties on dose delivery. The conversion of the dose to the chamber cavity to the dose to water becomes uncertain; and the geometrical details of the chamber, as well as the details of the delivery, are expected to be significant. In this study, a realistic model of the Exradin® A12 Farmer chamber is simulated. A framework is applied for the calculation of ionization chamber response to arbitrarily modulated fields as a summation of responses to pencil beams. This approach is used with the chamber model and tested against measurements in static open fields and dynamic MLC IMRT fields. As a benchmark test of the model, quality conversion factors values calculated by Monte-Carlo simulation with the chamber model are in agreement within 0.1 % and 0.4 % with those in the AAPM TG-51, for 6 MV and 18 MV photon beams, respectively. Pencil-beam kernels show a strong dependence on the geometrical details of the chamber. Kernel summations with open fields show a relative agreement within 4.0 % with experimental data; the agreement is within 2.0 % for dynamic MLC IMRT beams. Simulations show a strong sensitivity of chamber response on positioning uncertainties, sometimes leading to dose uncertainties of 15 %.

Sci‐YIS Fri ‐ 02: Calculated Pwall values in clinical photon beams

Dosimetry of high‐energy photon beams is based upon absorbed dose to water standards and requires the use of ionization chambers with several correction factors. This study investigates the wall correction factor, Pwall, in high‐energy photon beams for both cylindrical and parallel‐plate chambers using Monte Carlo calculations. For cylindrical chambers, dosimetry protocols use an empirical formula to determine Pwall despite high quality experimental evidence that there are problems with this formula. Wall corrections are not provided for parallel‐plate chambers in photon beams due to a lack of information available regarding the correction factors for these chambers. Monte Carlo calculations are carried out using the EGSnrc user‐code CSnrc to calculate the wall correction factor for a series of ion chambers using a correlated sampling variance reduction technique. Calculations of the wall correction are performed for a variety of chambers at the reference depth in photon beams, using realistic beam spectra from clinical accelerators, ranging in nominal energy from to 24 MV. For cylindrical chambers, Pwall values differ by as much as 0.8% from the predicted values. This discrepancy is used to resolve previous experimental results that pointed to problems with the Pwall formalism. Pwall values are also shown for parallel‐plate chambers in high‐energy photon beams and have corrections up to 2%. These data should allow parallel‐plate chambers to be used in photon as well as in electron beams.

SU‐CC‐J‐6C‐09: Calculated Pwall Values in Clinical Electron Beams

Purpose: Dosimetry of high‐energy electrons beams is based upon absorbed dose to water standards and requires the use of ionization chambers with several correction factors. There is little information regarding the details of many of these correction factors for electron beam dosimetry. This study investigates the wall correction factor, Pwall, in high‐energy electron beams for both cylindrical and parallel‐plate chambers using Monte Carlo calculations. Dosimetry protocols use a wall correction factor of unity in high energy electron beams, despite some evidence that there may be an effect greater than 1%. Method and Materials: Monte Carlo calculations are carried out using the EGSnrc system. In particular, the user‐code CSnrc is used to calculate the wall correction factor for a series of ion chambers using a correlated sampling variance reduction technique. The wall correction is computed as the ratio of doses to the air cavity for a chamber having a wall made entirely of water to that having a realistic chamber geometry. Calculations of the wall correction are performed for a variety of chambers at the reference depth in electron beams, using realistic electron beam spectra from clinical accelerators, ranging in nominal energy from 5 MeV to 25 MeV. Results: For parallel‐plate chambers, the wall correction is between 1.5% and 1.8% at the lower energies and varies from 0.5% to 1% at the highest energies. For cylindrical chambers, the wall corrections are up to 0.7% for the energy range investigated. Conclusion: EGSnrc calculations of the wall correction factors for ion chambers in high energy electron beams show that this effect is, in many cases, greater than 1%. This differs significantly from dosimetry protocols, which assume a correction of unity in these beams. Chamber‐specific values of the wall correction for parallel‐plate chambers are parametrized as a function of the beam quality.

Clinical impact of the detector size effect in 3D-CRT

Objective The detector size artificially increases the measured penumbra of radiotherapy fields. The aim of this work is to determine the influence of the detector size when planning three-dimensional conformal radiation therapy (3D-CRT) treatments. Material and methods Two anatomical sites of interest in 3D-CRT were studied: prostate and hypophysis chordoma. Conventional 3D-CRT treatments for two cases in these locations were planned with a FOCUS 4.0.0 (Computerized Medical Systems, USA) treatment planning system (TPS) equipped with Fast Fourier Transform Convolution (FFTC) and Multigrid Superposition (MGS) algorithms, making use of beams modelled from radiation profiles measured either with a 2.0 mm diameter detector (PFD 3G diode) or with a 5.5 mm diameter detector (PTW-31002 ionisation chamber). These detectors cover up the range of detector sizes commonly used to measure radiation profiles for 3D-CRT. Dose–volume histograms (DVHs), radiobiological indexes, tumor control probability (TCP) and normal tissue complication probability (NTCP) were analysed and compared for planning target volumes (PTVs) and organs at risk (OAR) studied. Results Important differences in DVH were found. OAR received higher dose levels when a 5.5 mm detector was used to measure profiles compared to the case in which a 2.0 mm detector was used. A 2 Gy increment in the mean rectal dose was found when the larger detector was used. In the same way, NTCP of brain stem in hypophysis chordoma treatments was doubled when this detector was used. Conclusion The current use of ionisation chambers of about 5 mm active diameter to get the necessary data to model treatment machines in radiotherapy treatment planning systems (TPS) implies a significant overirradiation of OAR close to the PTV in 3D-CRT treatments due to errors in the measured penumbra of beam profiles. To avoid this overirradiation, the measured profiles should either being acquired with a suitable detector size (2–3 mm active diameter) or being deconvoluted.

Recombination factors for the cylindrical FC65-G ionization chamber in pulsed photon beams and the plane-parallel Roos ionization chamber in pulsed electron beams

The use of ionization chambers in linac radiotherapy dosimetry requires various corrections to the measured charges, one of these being the recombination correction. The recombination correction factor (ks) is generally estimated from the two-voltage analysis (TVA) for each beam quality. However, it is possible that the ionization chamber above some threshold polarizing voltage does not follow the accepted Boag theory very well. Secondly the TVA is time-consuming as the chamber needs to stabilize after each polarizing voltage change and since it must be performed for each beam quality. Another approach consists in using the fact that ks is predicted to depend linearly on dose per pulse by Boag theory: determining this relationship once and for all using a multi-voltage analysis (MVA), one also checks the range validity of the Boag theory for the chamber considered. This work presents a thorough analysis of ks dependence on dose per pulse of FC65-G (cylindrical) and Roos (plane-parallel) ionization chambers in pulsed photon and electron beams, respectively. Within the uncertainties, the recombination factors are found to be independent of beam quality, and no deviation from the Boag theory is observed within the tested range of polarizing voltages. Before adapting the equations given using the MVA other users should check that their ionization chambers show the same dose per pulse dependence using the TVA for a few beam qualities.

Comparative study of polyacrylamid gels and thermoluminescent dosimeters used in external radiotherapy

Applications of polyacrylamid gels present a great interest in the development of 3D experimental dosimetry. The expansive application of this material will increase possibilities of dose control in external radiotherapy and contribute to increased precision of medical treatments. To develop this material, the dose response of the polyacrylamid gels was compared with that of other methods using detectors and simulations, such as an ionisation chamber, thermoluminescent dosimeters, and Monte-Carlo simulations (BEAM code). The curves of dose response in water, corresponding to a typical clinical situation, were compared for polyacrylamid gels and a cylindrical ionisation chamber ( 1.125 cm 3 ) under the same conditions. Three photon energies, X25, X15 and X6, from a linear accelerator SATURNE 43 were tested. A good correlation was shown with maximal differences between compared curves down to 3%. Polyacrylamid gels were used in the presence of radiation field heterogeneities, using 60Co. The polyacrylamid gels were compared with thermoluminescence dosimeters inside a polyvinylidene fluoride phantom. The latter presents a density close to bone and a volume similar to that of a human skull. The dimensions of the phantom do not allow the simple use of ionisation chambers, and therefore we opted for thermoluminescent dosimeters of the type “GR200A”, and for Monte-Carlo simulations. The results obtained under identical conditions inside the phantom showed good agreement between these different methods.

LiF:W as a scintillator for dosimetry in diagnostic radiology

The use of ionization chambers in diagnostic radiology is not feasible in measurement situations requiring small and robust dose sensors. The composition of LiF:W developed as a scintillator for the measurement of thermal neutrons seems profitable for an application in dosimetry of low-energy photons. Properties of a small LiF:W scintillator were determined with DV- and DH-standard radiation qualities. For a tube voltage range of 40 to 150 kV (corresponding to a HVL of 1.56 to 13.69 mm Al) a maximum variation in sensitivity of ±13% was determined for a scintillator thickness of 3.7 mm. The scintillator signal was linear in a range from 6.6 mGy min−1 to at least 13.7 Gy min−1. Higher dose rates could not be obtained in the measurement setup. The temperature dependence of the luminescence response was found to decrease from +3.7% (at +2.5 °C) to −7.0% (at +45 °C) with respect to the luminescence response at 20 °C. LiF:W appears to be an interesting choice as a dosimetric scintillator in diagnostic radiology making immediate measurements of dose and dose rate with high spatial resolution feasible.

The use of ionisation chambers for dose rate measurements at industrial irradiation plants

The use of ionisation chambers to measure dose rate at industrial irradiation plants has been studied as part of a wider project on real time dosimetry. The characteristics required of such a chamber are discussed. These include the ability to withstand operation at high cumulative doses (up to 5 MGy) and dose rates of up to about 150 kGy h −1. Other desirable features are water equivalence and immunity to environmental conditions such as temperature, pressure and humidity. A number of chambers have been assessed experimentally and a suitable chamber selected. The dosimetric characteristics of the chosen chamber have been assessed by comparison with absorbed dose measurements made using chemical dosimeters.

KQ factors for ionization chamber dosimetry in clinical proton beams

We discuss a formalism for clinical proton beam dosimetry based on the use of ionization chamber absorbed dose-to-water calibration and beam quality correction factors. A quantity kQ, the beam quality correction factor, is defined which corrects the absorbed dose-to-water calibration factor ND,w in a reference beam of quality Q0 to that in a user's beam of quality Q1. This study of proton beam quality correction factors used 60Co (kQ gamma) and proton (kQp) reference beams. The kQ gamma factors were measured using combined water calorimetry and ionometry for PTW and Capintec-Farmer-type ionization chambers, and were computed from standard dosimetry protocols. Agreement between measured and calculated kQ gamma values for both chambers was found within 1.2% in the plateau region for a monoenergetic 250-MeV beam and within 1.8% at the spread-out Bragg peak for a 155-MeV range-modulated beam. Comparison of absorbed doses to water determined in the range-modulated 155-MeV beam was performed with the PTW chamber using three calibration methods: Ngas calibration (AAPM Report 16), ND,w,gamma calibration in a 60Co beam in conjunction with a kQ gamma factor, and ND,w,p calibration in a proton beam in conjunction with a kQp factor. Absorbed doses to water obtained with the three methods agreed within 2% when ionization chamber dosimetry data were analyzed using the proton W-value for air from the AAPM Report 16 and the ICRU 49 proton stopping powers. The use of the proton-calibrated reference ionization chamber, in conjunction with the beam quality correction factor kQp, significantly reduced the systematic uncertainty of the absorbed dose determination.